The paramagnetic, heterometallic manganese cubanes [{E2(NCy)4}(MnCp)2] (Cy = C6H11, Cp = C5H5, E = As, Sb)

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The paramagnetic, heterometallic manganese cubanes [{E2(NCy)4}(MnCp)2] (Cy = C6H11, Cp = C5H5, E = As, Sb)† Alan Bashall,a Michael A. Beswick,b Helmut Ehlenberg,c Sara J. Kidd,b Mary McPartlin,a Julie S. Palmer,b Paul R. Raithby,b Jeremy M. Rawsonb and Dominic S. Wright*b Published on 13 April 2000. Downloaded by Christian Albrechts Universitat zu Kiel on 23/10/2014 05:47:04.

a

School of Chemistry, University of North London, London, UK N7 8DB Chemistry Department, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW. E-mail: [email protected] c Department of Physics, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, UK CB3 OHE b

Received (in Cambridge, UK) 17th December 1999, Accepted 23rd March 2000 Published on the Web 13th April 2000 The heterometallic cubanes [{E2(NCy)4}(MnCp)2] (E = Sb 1, As 2; Cy = C6H11, Cp = C5H5) are the first examples of complexes in which a paramagnetic metal ion has been incorporated into a p block ligand framework.

Mn–N bond lengths within these units (ca. 2.20 Å in both complexes9), this ring is far more puckered in 2 (the angle between the MnN2 planes being 161.9° in 1 and 142.6° in 2).

The coordination chemistry of ligand systems based on p block element imido frameworks has become increasingly studied in recent years.1,2 Species such as the homologous group 15 dianions [E2(NR)4]2– (E = P–Bi)1,3 offer readily accessible building blocks for the assembly of cage complexes containing a broad variety of mixed p block element/main group and transition element compositions.1a However, so far studies of the coordination chemistry of imido p block ligands have only involved main group and transition metal ions which possess closed-shell electronic configurations (for the transition metals d0 or d10 only have been reported).1,4,5 A current interest of ours involves utilising the differing geometric and bonding demands in homologous ligands of this type to control the degree of magnetic and/or bonding interactions between the ligandsupported metal frameworks present in these species.4c The heterometallic complexes [{Sb2(NCy)4}(MnCp)2] 1 and [{As2(NCy)4}(MnCp)2] 2, presented here, provide the first opportunity to investigate ligand control of magnetic properties in these systems. [{Sb2(NCy)4}(MnCp)2] 1 and [{As2(NCy)4}(MnCp)2] 2 were prepared by the nucleophilic substitution reactions of MnCp26 with [{E2(NCy)4}2M4] (E = Sb, M = Na;6 E = As, M = Li3b), with the elimination of CpM (Scheme 1.‡ 1H NMR studies of both showed only broad ill defined resonances which are consistent with paramagnetic materials. The low-temperature X-ray structures of 1 (Fig. 1) and 2 (Fig. 2)§ reveal that both complexes have similar pseudo-cubane structures in the solid state, which are composed of two (formally 17e) CpMn+ fragments7 complexed by [Sb2(NCy)4]22 and [As2(NCy)4]22 dianions. The similarity of the endo- and exo-cyclic Sb–N and As–N bonds of their [Sb2(NCy)4]22 and [As2(NCy)4]22 anions with those observed in the precursor Na complexes [{Sb2(NCy)4}2Na4]8 and [{As2(NCy)4}2Na4]4c illustrates that, like these complexes, the anion units have the primary role in dictating the overall structures of 1 and 2. The reduction in the endo-cyclic Sb–N and As–N bond lengths from 1 to 2 (of ca. 0.19 Å) has a major affect on the Mn2N2 rings in both complexes. In order to retain similar

Fig. 1 Structure of the Sb2Mn2 cubane 1 with H-atoms omitted for clarity. Key bond lengths (Å): Sb(1)–N(1) 2.109(5), Sb(1)–N(1A) 2.128(5), Sb(1)–N(2) 2.039(5), N(1)–Mn(1) 2.269(6), N(2)–Mn(1) 2.215(5), N(2)– Mn(1A) 2.217(5), Mn(1)–Mn(1A) 3.041(2), Mn–C range 2.421(7)-2.615(8) (Cpcentroid–Mn 2.16). Bond angles (°); N(1)–Sb(1)–N(1A) 80.1(2), Sb(1)– N(1)–Sb(1A) 99.9(2), N(1,1A)–Sb(1)–N(2) mean 88.2, Sb(1)–N(2)– Mn(1,1A) mean 97.6, Sb(1,1A)–N(1)–Mn(1) mean 93.2, N(1)–Mn(1)– N(2,2A) mean 80.7, Mn(1)–N(2)–Mn(1A) 87.6(2), N(2)–Mn(1)–N(2A) 91.0(2). Dihedral angles (°) between planes; MnN2 161.9, SbN2 178.4.

Scheme 1

Fig. 2 Structure of the As2Mn2 cubane 2 with H-atoms omitted for clarity. Key bond lengths (Å): As(1)–N(1A) 1.940(8), As(1)–N(1C) 1.816(7), As(1)–N(1D) 1.912(7), As(2)–N(1A) 1.923(7), As(2)–N(1B) 1.820(7), As(2)–N(1D) 1.935(7), Mn(1)–N(1B) 2.202(7), Mn(1)–N(1C) 2.170(7), Mn(1)–N(1D) 2.264(7), Mn(2)–N(1A) 2.278(7), Mn(2)–N(1B) 2.199(7), Mn(2)–N(1C) 2.253(7), Mn(1)…Mn(2) 2.934(2), Mn–C range 2.39(1)–2.53(1) (Cpcentroid–Mn 2.16). Mean angles (°): N(1A)–As(1,2)– N(1D) 83.0, N(1C)–As(1)–N(1A,1D) 91.9, N(1B)–As(2)–N(1A,1D) 91.3, Mn(1)–N(1B,1C)–Mn(2) 83.4, N(1C)–Mn(1,2)–N(1B) 90.8. Dihedral angles (°) between planes; MnN2 142.6, AsN2 176.2.

† Electronic supplementary information (ESI) available: plots of the effective magnetic moments of 1 and 2 as a function of temperature. See http://www.rsc.org/suppdata/cc/a9/a910196i/

DOI: 10.1039/a910196i

Chem. Commun., 2000, 749–750 This journal is © The Royal Society of Chemistry 2000

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Although this distortion has only a small affect on the Mn–N– Mn and N–Mn–N angles within the Mn2N2 ring units, a consequence of the presence of the more compact [As2(NCy)4]22 ligand in 2 is a large reduction in Mn…Mn separation [from 3.041(2) Å in 1 to 2.934(2) Å in 2]. The Mn…Mn separation in 2 is almost identical to that in Mn2(CO)10 (2.93 Å).10 Whilst Mn(II) ions are almost invariably high spin, the large ligand field associated with the p-donor Cp2 ligand makes the high- and low-spin electronic configurations energetically similar. Indeed, for manganocenes (MnCpR2; CpR = substituted Cp) examples of pure low-spin, pure high-spin and high/low-spin equilibria have been observed (depending on the donor or acceptor character and steric demands of ring substituents).11,12 For the half-sandwich (MnCpR) derivatives, pure high-spin behaviour has been observed.13 For 1 and 2, the possibility of high-/low-spin equilibria is complicated by the potential for superexchange via the bridging imido ligands. Preliminary magnetic studies of 1 and 2 (5.5–380 K) show a marked temperature dependence of meff (ESI†). Even at 380 K, the observed moments (6.0 and 6.6 mB for 1 and 2, respectively) are significantly less than that expected for two high-spin Mn(II) ions per molecule (8.37 mB). The value of meff decreases steadily on cooling, reaching 2.6 mB for 1 and 1.9 mB for 2 at 5.5 K. While the value for 1 is consistent with two non-interacting lowspin Mn(II) ions (2.5 mB), the significantly lower value for 2 may indicate intramolecular exchange between the Mn ions. Attempts to model the data for 1 and 2 have so far proved inconclusive.¶ The suggestion that a high-/low-spin equilibrium may be occurring over a wide temperature range for both complexes is apparently inconsistent with the long Mn–C bond lengths in 1 and 2 [range 2.389(10)–2.615(8) Å], which are typical of high-spin manganocenes (cf. ca. 2.11–2.14 Å in lowspin species).12 Although some shortening of the Mn–C bonds is anticipated for the high-/low-spin equilibrium at the temperatures at which the structures of the complexes were obtained (180 K for 1 and 223 K for 2), a possible reason for the maintenance of long metal–ligand bonds in both complexes is the sterically congested nature of the [E2(NCy)4]22 ligand frameworks (making closer approach of the Cp ligands unfavourable12b). A detailed study of the magnetic behaviour (EPR and solid-state susceptibility) and variable-temperature X-ray diffraction studies of 1 and 2 will be the subject of a full paper. We gratefully acknowledge the EPSRC (A. B., S. J. K., J. S. P., M. McP., P. R. R., D. S. W.) and The Leverhulme Trust (M. A. B.) for financial support. We also thank Dr J. E. Davies for collecting X-ray data on 1. We are also particularly grateful to the referees for their helpful comments regarding the magnetic behaviour of 1 and 2.

Notes and references ‡ Syntheses: 1: solid MnCp2 (0.273g, 1.48 mmol) and [{Sb2(NCy)4}2Na4] (0.676 g, 0.74 mmol) were mixed together in a glove box under argon. The mixture was dissolved in toluene (20 ml) and brought to reflux, producing a fine gelatinous solid of CpNa which was separated by the addition of thf (10 ml) followed by filtration. The red filtrate was reduced to dryness under vacuum and the solid residue dissolved by the addition of hexane (10 ml) and toluene (10 ml) and gentle heating. Storage (25 °C, 12 h) gave red plates of 1. Yield 0.520 g (81%). Decomp. ca. 200 °C. dH(+25 °C, thf-d8, 250 MHz), 4.75 (Cp), 1.8–0.7 (Cy groups) (Found: C 46.2; H 6.0; N 5.3. Calc. for 1: C, 46.8; H, 6.2; N, 6.4%). 2: [{As2(NCy)4}2Li4] was prepared in situ by the reaction of As(NMe2)3 (5.0 ml, 2.43 mol dm–3 solution in toluene, 12.15 mmol) with a solution of CyNH2 (1.39 ml, 12.15 mmol) in toluene (8 ml), followed by the addition of a suspension of CyNHLi (12.15 mmol) in toluene (8 ml). MnCp2 (2.25 g, 12.15 mmol) in thf (10 ml) was added to this mixture and a white precipitate of CpLi was formed after bringing to reflux. Filtration gave a yellow solution which was stored (25 °C, 12 h) to give yellow plates of 2. Yield 1.62 g (34%). Decomp. ca. 210 °C. dH(+25 °C, thf-d8, 250 MHz), ca.

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6.5 (Cp), 2.5–0.5 (Cy groups) (Found: C, 51.5; H, 6.6; N, 6.2. Calc. for 2: C, 52.4; H, 6.9, N, 7.2%). § Crystal data: for 1: C34H54Mn2N4Sb2, M = 872.20, tetragonal, space group P41212, Z = 4, a = 11.775(2), c = 26.095(5) Å, U = 3618.1(11) Å3, Dc = 1.601 g cm–3, F(000) = 1752, m(Mo–Ka) = 2.187 mm21, T = 180(2) K. Data in the q range 1.90–25.06° were collected on a Nonius Kappa CCD diffractometer. The structure was solved by direct methods. Full-matrix least-squares refinement on F2 with 190 parameters was based on 3156 independent data (total collected 5231, Rint = 0.044). Final R1 = 0.028 [I > 2s(I)], wR2 = 0.126 (all data).14 Largest peak and hole in the final difference map 0.808 and 21.750 e Å23. Absolute structure parameter 20.09(6). For 2: C34H54As2Mn2N4, M = 778.53, monoclinic, space group P21/c, Z = 4, a = 10.336(2), b = 17.095(6), c = 20.506(5) Å, b = 103.99(2)°, U = 3516(2) Å3, Dc = 1.471 g cm23, F(000) = 1608, m(Mo-Ka) = 2.615 mm21, T = 223(2) K. Data in the q range 2.03–23.01° were collected on a Siemens P4 difractometer, and corrected for absorption effects using Y scans. The structure was solved by direct methods. Full-matrix least-squares refinement on F2 with 379 parameters was based on 4893 independent data (total collected 6294, Rint = 0.0509). Final R1 = 0.054 [I > 2s(I)], wR2 = 0.156 (all data).14 Largest peak and hole in the final difference map 0.868 and 20.877 e Å23. CCDC 182/1581. See http://www.rsc.org/suppdata/cc/a9/a910196i/ for crystallographic files in .cif format. ¶ Magnetic behaviour of 1 and 2: solid samples were measured between 5.5 and 380 K using a SQUID magnetometer in fields of 100 (for 1) and 1000 G (for 2). They were corrected for diamagnetism using Pascals’ constants. Attempts to model the magnetic behaviour as a low-spin ground state with a thermally accessible high-spin state were unsuccessful as was an attempt to fit the data to two interacting Heisenberg S = 5⁄2 ions. For 1, a high-spin/ low-spin thermal equilibrium fits the data throughout the temperature range 5.5-380 K, and also for the range 25–380 K for 2. 1 (a) M. A. Beswick and D. S. Wright, Coord. Chem. Rev., 1998, 176, 373; (b) M. A. Beswick, M. E. G. Mosquera and D. S. Wright, J. Chem. Soc., Dalton Trans., 1998, 2437, and references therein. 2 For related Group 16 examples, see: R. Fleischer, S. Freitag, F. Pauer and D. Stalke, Angew. Chem., Int. Ed. Engl., 1996, 35, 204; T. Chivers, X. Gao, M. Parvez and G. Schatte, Inorg. Chem., 1996, 35, 4094. 3 I. Schranz, L. Stahl and R. J. Staples, Inorg. Chem., 1998, 37, 1493. 4 Group 15 examples, see (a) D. F. Moser, I. Schranz, M. C. Gerrety, L. Stahl and R. J. Staples, J. Chem. Soc., Dalton Trans., 1999, 751; (b) L. Grocholl, V. Huch, L. Stahl, R. Staples, P. Steinhart and A. Johnson, Inorg. Chem., 1997, 36, 4451; (c) A. Bashall, M. A. Beswick, E. A. Harron, A. D. Hopkins, S. J. Kidd, M. McPartlin, P. R. Raithby, A. Steiner and D. S. Wright, Chem. Commun., 1999, 1145; (d) L. Grocholl, I. Schranz, L. Stahl and R. J. Staples, Inorg. Chem., 1998, 37, 2496. 5 Group 16 examples, see: T. Chivers, T. Parvez, M. Schatte and G. P. A. Yap, Inorg. Chem., 1999, 38, 1380; 6 For further aspects of the nucleophilic substitution of Cp2Mn, see: K. Jonas, Angew. Chem., Int. Ed. Engl., 1985, 24, 295. 7 Electron count for 1 and 2, viewed as [E2(NCy)4]2–·2Mn2+; Mn(II) (5e), Cp– (6e), E–(m-N)–E (2e), Mn–(m-N)–Mn (4e). 8 A. Bashall, M. A. Beswick, C. N. Harmer, A. D. Hopkins, M. McPartlin, M. A. Paver, P. R. Raithby and D. S. Wright, J. Chem. Soc., Dalton Trans., 1998, 1389. 9 These Mn–N bonds are considerably longer than in related Mn(II) complexes, for example see N. Wiberg, H.-W. Häring, G. Gottfried and P. Friedrich, Chem. Ber., 1978, 111, 2708; W. J. Grigsby and P. P. Power, J. Chem. Soc., Dalton Trans., 1996, 4613. 10 M. R. Churchill, K. N. Amoh and H. J. Wasserman, Inorg. Chem., 1981, 20, 1609; M. Martin, B. Rees and A. Mitschler, Acta Crystallogr., Sect. B, 1982, 38B, 6. 11 N. Hebendanz, F. Köhler, G. Müller and J. Riede, J. Am. Chem. Soc., 1986, 108, 3281. 12 (a) M. E. Switzer, R. Wang, M. F. Rettig and A. H. Maki, J. Am. Chem. Soc., 1974, 96, 7669; (b) M. L. Hays, D. J. Burkey, J. S. Overby, T. P. Hanusa, S. P. Sellers, G. T. Yee and V. G. Young, Jr., Organometallics, 1998, 17, 5521 and references therein. 13 F. H. Köhler, N. Hebendanz, U. Thewalt, B. Kanellakopulus and R. Klenze, Angew. Chem., Int. Ed. Engl., 1984, 23, 721; J. Heck, W. Massa and P. Weinig, Angew. Chem., Int. Ed. Engl., 1984, 23, 722; F. H. Köhler, N. Hebendanz, G. Müller and U. Thewalt, Organometallics, 1987, 6, 115. 14 SHELXTL PC version 5.03, Siemens Analytical Instruments, Madison, WI, 1994.